 (def energy-as (as-f :ft-lb :J 1.35582))
 (def force-as (as-f :lbf :N 4.44822))
 (def length-as (as-f :m :ft 3.28084))
+(def mass-as (as-f :kg :lb 2.20462262185))
 (def power-as (as-f :hp :W 745.69987158227022))
 (def pressure-as (as-f :psi :kPa 6.894757293168361))
 (def proppant-concentration-as (as-f :lb-per-gal :kg-per-m3 119.826))

        measurement (generate-measurement-f unit)]
    [measurement (measurement-as-f measurement (first (remove (partial = unit) units)))]))

(generate-measurement-pair (set (quantity-unit-map :length))
                           rand-length-unit typical-stage-length length-as)

(defn rand-measurement
  ([]


(defn convert-units-f [from to]
  (let [factors   {[:ft-lb :J]                1.35582
                   ;; slurry rate conversion not needed (use [:bbl :m3])
                   [:m3 :bbl]                 6.28981077}
        converter {[:kg-per-m3 :lb-per-cu-ft] #(/ % (factors [:lb-per-cu-ft :kg-per-m3]))
                   [:ft-lb :J]                #(* % (factors [:ft-lb :J]))
                   [:J :ft-lb]                #(/ % (factors [:ft-lb :J]))
                   [:bbl :m3]                 #(/ % (factors [:m3 :bbl]))
                   [:m3 :bbl]                 #(* % (factors [:m3 :bbl]))}]
    (converter [from to])))
